Thanks for writing your query.
The swelling that you had mentioned could be a lymph
node swelling. It is difficult to say without seeing the swelling what it is. There is possibility that you had developed due to flu vaccine
. If it is painful, apply ice on the swelling & keep a watch on the size of swelling. If the swelling should decrease in 4-5 days , but if it is increasing in size & pain doesnot subside that do visit your local doctor.
Such type of lymph node sweling are seen after vaccination
Wish you good health.